Microwave dielectric analyzer

ABSTRACT

Various examples related to microwave dielectric analyzers and their use are provided. In one example, a microwave dielectric analyzer includes a measurement apparatus having a conductive electrode that can couple to a microwave analyzer and processing circuitry that can determine a dielectric characteristic of the dielectric specimen using a reflection coefficient measured by the microwave analyzer. The dielectric characteristic can be determined using a computational electromagnetic model of the measurement apparatus. The reflection coefficient can be measured by the microwave analyzer with the dielectric specimen in contact with the conductive electrode and/or sandwiched between conductive electrodes. The conductive electrodes can be axially aligned, and the second electrode may not be coupled to the microwave analyzer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to, and the benefit of, co-pending U.S.provisional application entitled “Microwave Dielectric Analyzer” havingSer. No. 62/699,910, filed Jul. 18, 2018, which is hereby incorporatedby reference in its entirety.

BACKGROUND

In many applications, material characterization is important toselecting and predicting design results. For example, printed circuit(PC) board characteristics can affect the operation of the circuitprinted on the PC board. In some cases, materials surrounding a wirelessantenna can affect the performance of that antenna. The characterizationof dielectric properties of such sheet materials over a wide frequencyrange can be expensive and time consuming.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present disclosure can be better understood withreference to the following drawings. The components in the drawings arenot necessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the present disclosure. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIGS. 1A-1C illustrate an example of a measurement apparatus of amicrowave dielectric analyzer, in accordance with various embodiments ofthe present disclosure.

FIG. 2 is a graphical representation showing a specimen under testsandwiched between the conducting electrodes of the measurementapparatus, in accordance with various embodiments of the presentdisclosure.

FIG. 3 is a plot illustrating examples of computationally modeled phaseof a reflection coefficient S11 for various dielectric specimens in adielectric analyzer apparatus, in accordance with various embodiments ofthe present disclosure.

FIGS. 4A and 4B are plots illustrating examples of computationallymodeled phase and amplitude for low and moderate loss specimens with athickness of 0.5 mm inserted into a dielectric analyzer apparatus, inaccordance with various embodiments of the present disclosure.

FIGS. 5A and 5B are plots illustrating examples of computationallymodeled phase and amplitude for low and moderate loss specimens with athickness of 3.0 mm inserted into a dielectric analyzer apparatus, inaccordance with various embodiments of the present disclosure.

DETAILED DESCRIPTION

Disclosed herein are various aspects related to microwave dielectricanalyzers, and methods thereof. The microwave dielectric analyzers canbe designed for non-destructive characterization of dielectric specimenssuch as, e.g., printed circuit (PC) boards, antenna substrates, andantenna radomes/packaging. The microwave dielectric analyzers canmeasure flat specimens (e.g., up to 3 mm thick or larger), and determinedielectric characteristics of the specimen with measurements taken in arange from about (or below) 3 MHz to about 6 GHz, about 10 MHz to about6 GHz, or more. Reference will now be made in detail to the descriptionof the embodiments as illustrated in the drawings, wherein likereference numbers indicate like parts throughout the several views.

Referring to FIG. 1A, shown is an example of a measurement apparatus (orfixture) 100 of a microwave dielectric analyzer. The measurementapparatus 100 can include a pair of parallel conductive electrodes formeasuring dielectric specimens secured between the conductiveelectrodes. For example, a first (or bottom) conductive electrode 103can be axially aligned with a second (or upper) conductive electrode106, such that measurements can be obtained for a dielectric specimensandwiched (or secured) between the conductive electrodes 103 and 106.The conductive electrodes 103 and 106 can be made of appropriateconductive materials (e.g., stainless steel, aluminum, etc.). One of theconductive electrodes is coupled to a microwave analyzer, while theother conductive electrode remains disconnected. A specimen under testcan be inserted between the conductive electrodes 103 and 106. As shownin FIG. 1A, a radio frequency (RF) connector 109 (e.g., an N-type, SMA,or other connector) can be used to couple the first electrode 103 to themicrowave analyzer.

The measurement apparatus (or fixture) 100 of the microwave dielectricanalyzer can also include a metal base plate 112 surrounding the firstelectrode 103, which can provide shielding and structural support forthe bottom electrode. The base plate 112 can be made of an appropriateconductive material (e.g., stainless steel, aluminum, etc.). Inaddition, the measurement apparatus 100 can include a non-conductiveyoke 115 supporting the second conductive electrode 106 over the firstconductive electrode 103. The yoke 115 can be made from plastic or otherappropriate insulating material (e.g., polystyrene, PPO, polyethylene,acrylic, ultem, etc.). In the example of FIG. 1A, the yoke 115 includesa support arm 118 positioned on the base plate, and a handle 121 that isthreaded through the support arm and can be turned to adjust theseparation between the first and second conductive electrodes 103 and106. By turning the handle 121, the second conductive electrode 106 canbe screwed down onto the specimen being tested.

Referring next to FIG. 1B, shown is a cross-sectional view of themeasurement apparatus (or fixture) 100 of FIG. 1A. As shown, the firstconductive electrode 103 can be mounted in the metal base plate 112using, e.g., a dielectric spacer 124 such as an insulator (e.g.,polystyrene, PPO, acrylic, etc.) or other appropriate material with theRF connector 109 extending through the opposite side of the base plate112. The second conductive electrode 106 can be attached to the handle121 of the yoke 115 using a fastener such as a nylon screw of otherappropriate fastener. In some embodiments, the second conductiveelectrode 106 can be integrated into the yoke 115 using othertechniques. The yoke 115 is configured to allow the position of thesecond conductive electrode 106 to be adjusted. For example, the supportarm 118 and handle 121 can be threaded 127 such that turning the handle121 can adjust the position of the second conductive electrode 106. Inthis way, the second conductive electrode 106 can be lowered onto thespecimen under test, sandwiching the specimen between the conductiveelectrodes 103 and 106.

FIG. 10 illustrates exemplary dimensions of the measurement apparatus(or fixture) 100 of FIGS. 1A and 1B. As can be seen, the diameter of thesecond conductive electrode is larger than the diameter of the firstconductive electrode 103. The sizes of the electrodes can be set tomaximize measurement sensitivity. Additionally, the maximum size of thebottom electrode can be set to prevent resonances in the apparatusresponse over the desired measurement frequency range. The diameter ofthe second conductive electrode 106 can overlap with a portion of themetal base plate 112. The larger second electrode size in this examplealso helps to reduce errors from electrode misalignment. In thisconfiguration, the measurement apparatus 100 can be used to testspecimens of various sizes. For example, a square specimen of about 12inches by 12 inches and having a thickness of about (or just under)0.125 inch or about 3 mm can be accommodated by the measurementapparatus 100. FIG. 2 is a graphical representation illustrating aspecimen 203 sandwiched between the conductive electrodes 103 and 106.As can be seen, the second conductive electrode 106 extends beyond thefirst conductive electrode 103, and overlaps a portion of the base plate112.

In other embodiments, the measurement apparatus 100 includes only asingle (or bottom) conductive electrode 103 without including the secondconductive electrode as illustrated in FIGS. 1A-1C. In theseimplementations, the measurement apparatus 100 can also include a metalbase plate 112 surrounding the single electrode 103, which can provideshielding for the single conductive electrode 103. The non-conductiveyoke 115, without the second conductive electrode 106, can be used tosecure the specimen under test in contact with the single conductiveelectrode 103. For example, the handle 121 can be turned to applypressure to the specimen to hold it against the conductive electrode103, and against a surface of the base plate 112. Other features of themeasurement apparatus 100 can also be discussed above.

As previously mentioned, the microwave dielectric analyzer can be usedto determine characteristics of flat dielectric specimens over a widerange of frequencies (e.g., about (or below) 3 MHz to about 6 GHz, about10 MHz to about 6 GHz, etc.). The measurement apparatus 100 can beadjusted to test specimens having different thicknesses. For example,the yoke 115 of the measurement apparatus 100 can be configured to allowtesting of planar specimens having a thickness of up to about 3 mm, upto about 0.125 inch, or more.

While suitable for non-destructive characterization a wide range ofdielectric specimens, one application of the microwave dielectricanalyzer is characterization of PC board specimens. For instance, a usercan place the specimen over the first (or bottom) conductive electrode103, and then adjust the yoke 115 to place the second (or upper)conductive electrode 106 on top of the specimen to make measurementsusing the microwave analyzer coupled to the first conductive electrode103. The microwave dielectric analyzer can use a vector network analyzer(VNA), vector voltmeter, or similar microwave device to obtain thereflection coefficient (S11) measurements, which can then be convertedto dielectric characteristics of the specimen (e.g., complexpermittivity including both real and imaginary components) using thecomputational model of the measurement apparatus 100. The VNA isconfigured to interface with processing circuitry such as a computingdevice (e.g., a personal computer, laptop, notepad or smartphone) orother appropriate circuitry. Such a determination method is unique usingthe fixture of the measurement apparatus 100.

The measurement fixture 100 can be optimized using computationalelectromagnetic (CEM) codes, and dimensions of the first and/or secondconductive electrodes 103/106 can be optimized for a desired frequencyrange, e.g., up to 6 GHz. FIG. 10 provides an example of dimensions andalignment of the conductive electrode 103 and 106 for a range from about3 MHz to about 6 GHz, or about 10 MHz to about 6 GHz. The first (orbottom) conductive electrode 103 does not have chamfers, but the second(or upper) conductive electrode 106 may have small chamfers. Dimensionsof the yoke 115, which serves to aid in electrode positioning, can varysince the support arm 118 and handle 121 are both made of non-conductingmaterials. No metal screws or other fasteners are above the base plate112, as they may cause small resonances in response that increase themeasurement uncertainty. Simulations have shown that a 2 mm gap betweenthe coax feed 109 and the first (or bottom) conductive electrode 103works well for preserving the impedance match for an N-type connectorwith air as the spacer. Other gap and electrode dimensions may workbetter for other types of microwave connectors.

The measured reflection coefficient S11 can be correlated withdielectric properties of the specimen being tested. The computationalmodel may be constructed using a conventional electromagnetic code, suchas FEM (finite element method) or FDTD (finite difference time domain).The dielectric properties can be determined based upon a computationalmodel. The computational model can be used to create a pre-calculatedlook-up table, which can be searched to identify the dielectricproperties from the measured reflection coefficient of the measurement(or fixture) apparatus 100. A series of full-wave simulations can be runfor different combinations of sample dielectric permittivity, dielectricloss, and thickness. This data can then be used to form the basis fortranslating the measured reflectivity (phase and amplitude) into complexpermittivity. Interpolation can be used in the inversion to obtain highfidelity results from a courser database of simulated results.

Referring to FIG. 3, shown is a plot illustrating the computationallymodeled phase of the S11 over a frequency range from about 1 MHz to 6GHz. FIG. 3 shows the computationally modeled phase of the S11 from themeasurement assembly 100 as a function of three different dielectricspecimens. Computationally modeled amplitudes are also determined forthe different dielectric specimens. The computationally modeled phaseand amplitude can be used to determine the dielectric characteristicsbased upon the measured reflection coefficient.

FIGS. 4A and 4B show examples of computationally modeled S11 for variouslow and moderate loss specimens having a thickness of 0.5 mm. FIG. 4Ashows the amplitude over a frequency range of about 1 MHz to about 6GHz, and FIG. 4B shows the phase over the same frequency range. FIGS. 5Aand 5B show examples of computationally modeled S11 for the low andmoderate loss specimens having a thickness of 3.0 mm, with FIG. 5Ashowing the amplitude and FIG. 5B showing the phase over the frequencyrange. A database of these responses can be used to determine thecomplex permittivity (or other dielectric characteristics) from the S11measured using the measurement apparatus 100. Backwards calculations canbe used to identify the dielectric properties of the specimen undertest.

It should be emphasized that the above-described embodiments of thepresent disclosure are merely possible examples of implementations setforth for a clear understanding of the principles of the disclosure.Many variations and modifications may be made to the above-describedembodiment(s) without departing substantially from the spirit andprinciples of the disclosure. All such modifications and variations areintended to be included herein within the scope of this disclosure andprotected by the following claims.

The term “substantially” is meant to permit deviations from thedescriptive term that don't negatively impact the intended purpose.Descriptive terms are implicitly understood to be modified by the wordsubstantially, even if the term is not explicitly modified by the wordsubstantially.

It should be noted that ratios, concentrations, amounts, and othernumerical data may be expressed herein in a range format. It is to beunderstood that such a range format is used for convenience and brevity,and thus, should be interpreted in a flexible manner to include not onlythe numerical values explicitly recited as the limits of the range, butalso to include all the individual numerical values or sub-rangesencompassed within that range as if each numerical value and sub-rangeis explicitly recited. To illustrate, a concentration range of “about0.1% to about 5%” should be interpreted to include not only theexplicitly recited concentration of about 0.1 wt % to about 5 wt %, butalso include individual concentrations (e.g., 1%, 2%, 3%, and 4%) andthe sub-ranges (e.g., 0.5%, 1.1%, 2.2%, 3.3%, and 4.4%) within theindicated range. The term “about” can include traditional roundingaccording to significant figures of numerical values. In addition, thephrase “about ‘x’ to ‘y’” includes “about ‘x’ to about ‘y’”.

Therefore, at least the following is claimed:
 1. A microwave dielectricanalyzer, comprising: a measurement apparatus having: a first conductiveelectrode configured to couple to a microwave analyzer; and a secondconductive electrode axially aligned with the first conductiveelectrode, where the second conductive electrode is not coupled to themicrowave analyzer; and processing circuitry configured to determine adielectric characteristic of a dielectric specimen based upon areflection coefficient measured by the microwave analyzer with thedielectric specimen sandwiched between the first and second conductiveelectrodes, the dielectric characteristic determined based upon acomputational electromagnetic model of the measurement apparatus.
 2. Themicrowave dielectric analyzer of claim 1, wherein the dielectriccharacteristic is permittivity, loss or conductivity of the dielectricspecimen.
 3. The microwave dielectric analyzer of claim 1, wherein thedielectric characteristic is determined from a look-up table determinedusing the computational electromagnetic model of the measurementapparatus.
 4. The microwave dielectric analyzer of claim 3, wherein thedielectric characteristic is determined by interpolating the look-uptable.
 5. The microwave dielectric analyzer of claim 1, wherein thedielectric characteristic of the dielectric specimen is determined forfrequencies above 1 GHz.
 6. The microwave dielectric analyzer of claim1, wherein the reflection coefficient is measured in a range from about3 MHz to about 6 GHz.
 7. The microwave dielectric analyzer of claim 1,wherein the measurement apparatus is calibrated with a singlecalibration measurement of a known calibration specimen, without theneed for additional calibration measurements.
 8. The microwavedielectric analyzer of claim 7, wherein the single calibrationmeasurement is taken after measurement of the dielectric specimen. 9.The microwave dielectric analyzer of claim 1, wherein the position ofthe second conductive electrode is adjustable to sandwich the dielectricspecimen between the first and second conductive electrodes.
 10. Themicrowave dielectric analyzer of claim 1, wherein the measurementapparatus comprises a metal base plate surrounding the first conductiveelectrode.
 11. The microwave dielectric analyzer of claim 10, whereinthe measurement apparatus comprises an insulating yoke that mechanicallysupports the second conductive electrode, the insulating yoke configuredto axially adjust the position of the second conductive electrode withrespect to the first conductive electrode to sandwich the dielectricspecimen between the first and second conductive electrodes.
 12. Amicrowave dielectric analyzer, comprising: a measurement apparatushaving a single conductive electrode configured to couple to a microwaveanalyzer; and processing circuitry configured to determine a dielectriccharacteristic of a dielectric specimen based upon a reflectioncoefficient measured by the microwave analyzer with the dielectricspecimen in contact with the single conductive electrode, the dielectriccharacteristic determined based upon a computational electromagneticmodel of the measurement apparatus.
 13. The microwave dielectricanalyzer of claim 12, wherein the dielectric characteristic ispermittivity, loss or conductivity of the dielectric specimen.
 14. Themicrowave dielectric analyzer of claim 12, wherein the dielectriccharacteristic is determined from a look-up table determined using thecomputational electromagnetic model of the measurement apparatus. 15.The microwave dielectric analyzer of claim 14, wherein the dielectriccharacteristic is determined by interpolating the look-up table.
 16. Themicrowave dielectric analyzer of claim 12, wherein the measurementapparatus comprises a metal base plate surrounding the single conductiveelectrode.
 17. The microwave dielectric analyzer of claim 16, whereinthe measurement apparatus comprises an insulating yoke configured tosecure the dielectric specimen in contact with the single conductiveelectrode.
 18. The microwave dielectric analyzer of claim 17, whereinthe dielectric specimen is in contact with the metal base plate when incontact with the single conductive electrode.
 19. The microwavedielectric analyzer of claim 12, wherein the dielectric characteristicof the dielectric specimen is determined for frequencies above 1 GHz.20. The microwave dielectric analyzer of claim 12, wherein themeasurement apparatus is calibrated with a single calibrationmeasurement of a known calibration specimen, without the need foradditional calibration measurements.